Devices of the previously-described type are already known in the state of the art. Thus, the patent Fr-A-2 617 694 describes a device for packaging two substances, comprising two cylinders which are superposed and joined together by their respective bottom, particularly by molding so that they form just one piece, in such a way that the caps are at opposite ends from one another. However, one cylinder is made taller than the other, which makes them expensive and complicated to manufacture. Furthermore, these cylinders are not intended to be separated after they have been joined together.
Known elsewhere, from the patent FR-A-2 680 357, is a two-bottle packaging allowing two substances to be stored separately then mixed at the moment of use. Because the substances have to be mixed, the packaging comprises two bottles which are superposed but linked by an intermediate piece equipped with a hollow shaft. The lower bottle comprises a neck with fits into the hollow shaft and which pushes back a stopper so that the contents of the two bottles can be mixed. As can be seen, this two-bottle packaging meets a special need, which is that of mixing the substances, and therefore has a relatively complicated and expensive structure.